Referring to FIG. 1A, a two-phase buck converter includes two parallel connected converters, each including a first control switch S1, S2 and a shunt switch SH connected to one another at a switched node 10. An inductor 12 is connected to each respective switched node 10 at one pole thereof and to an output node 14 at another pole thereof. Inductors 12 in FIG. 1A are not magnetically coupled to one another, i.e., they do not share a common core.
FIG. 1B shows a two-phase buck converter in which inductors 12 are coupled to one another, which means that inductors 12 are inverse connected and wound about a common core.
It is known that current cancellation in a multi-phase voltage regulator such as a multi-phase buck converter can result in the reduction of ripple in the output current.
Current cancellation in a multi-phase buck converter can be extended to the inductors and the switches by multi-phase inductor coupling. For example, as illustrated in FIG. 1C, in a two-phase buck converter, implementing coupled inductors (FIG. 1B) can effectively reduce the peak to peak current that flows through the inductors. Thus, less steady state losses can be expected when coupled inductors are used without sacrificing dynamic performance. Moreover, compared to a multi-phase buck converter in which the inductors are not coupled (FIG. 1A), when the inductors are coupled, the size of the inductors can be reduced without inducing more ripple current. Thus, faster transient response can be expected without sacrificing converter efficiency.
A typical multi-phase converter according to the prior art can include more than two inductors wound around a common core. Such an arrangement is inherently asymmetrical meaning that its phases exhibit non-identical magnetic characteristics. The variation in the magnetic characteristics can lead to sub-harmonic output ripple.
It is desirable to reduce the sub-harmonic output ripple in a multi-phase converter.